function F=cond_surf(a,b,x,y)
epsi=1e-10;
if abs(x-0)<epsi || abs(y-0)<epsi
    F=0;
elseif abs(x-a)<epsi
    F=sin(3*pi*y/b);
elseif abs(y-b)<epsi
    F=sin(3*pi*x/a)*cos(pi*x/a);
else
    F=0;
end
